Separation of particulate material by the application of electric fields

ABSTRACT

A separator of particulate material by the use of electric fields is comprised of a pair of spaced plates of a dielectric material between which particulate material is arranged to be fed, the material being propelled in a given direction by a vibratory feeder attached to the lowermost plate. A set of parallel spaced electrodes are provided on each of said plates out of contact with the material and extending in a direction laterally of said given direction, and an AC voltage is applied between the sets of electrodes so that alternating electrical fields are set up at spaced locations along said plates in said given direction. Certain of said particulate material is caused to be repelled by said electrical fields and deflected thereby to move in said direction laterally of said given direction while the remainder of said material moves generally in said given direction.

United States Patent 161 Shook et al. 51March 13, 1973 1 1 SEPARATION OFPARTICULATE 3,249,225 5/1966 Stuetzer et all ..2o9 129 MATERIAL BY THEAPPLI OF 3,291,302 12/1966 Brastad..... ..209 129 x [75] Inventors: Paul1 R. Shook, Colorado Springs; Assistant Examiner Ra] v ph J. H111 EarlSweeney f Ralph AttmeyF. W. Anderson, C. E. Tripp and R. S. Duncan,Colorado Springs, all of CD 0 Kelly [73] Assignee: FMC Corporatiom'SanJose, Calif. [5 ABSTRAC [22] Filed: July 9, 1970 A separator ofparticulate material by the use of electric fields is comprised of apair of spaced plates of a [21] Appl 53518 dielectric material betweenwhich particulate material is arranged to be fed, the material beingpropelled in a [52] U.S. CI .209/130, 209/127, 209/223 R, givendirection by a vibratory feeder attached to the 209/226, 209/228lowermost plate. A set of parallel spaced electrodes [51] Int. Cl ..B03c7/04 are provided on each of said plates out of contact with [58] Fieldof Search ..209/127-l31, 223 R, the material and extending in adirection laterally of 209/223 A, 225,226, 228 said given direction, andan AC voltage is applied 1 between the sets of electrodes so thatalternating elec- [56] References Cited trical fields are set up atspaced locations along said plates in said given direction. Certain ofsaid particu- 1 UNITED STATES PATENTS late material is caused to berepelled by said electrical 1,179,937 4/1916 Kraus; .209 127 0 fieldsand deflected thereby to m in Said direction 1,355,477 /1920 Howell..209/l29 laterally of said given direction while the remainder of3,253, /1 fl i -2 said material moves generally in said given direction.

,869 1/1955 Gearu ....209/127 C 1 2,848,108 8/1958 Brastad et al..209/127 R 28 Claims, 9 Drawing Figures A vI! \1 A I RT? T 1 *(v KC":I\\\\\\\\{\\ Q 42 t 73 J 1k 60 .'ii2. 7 1l o l I 7)- 2' 7"") -E)-- 1-3fz ez I/ ;i 1 I .8 1 I I 1 I/ I x fa u! 4. .J 4 4 z .l z -1 v 1 [llELECTRIC FIELDS Primary Examiner-Tim R. Miles PATENTEUHAR 1 3197s SHEETl 0F 4 FIG EEI

TO SIDE ELECTRODES TO LOWER PLATE TO CENTER ELECTRODES ELECTRODES 1SEPARATION OF PARTICULATE MATERIAL BY THE APPLICATION OF ELECTRIC FIELDSBACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention pertains to apparatus for separating fine particulate materialin accordance with certain electrical characteristics of the material,and more particularly, it pertains to separators which utilize. highvoltage electrical fields to cause a separation of the individualparticles of a particulate material.

2. Description of the Prioi' Art The. separation of fine particulatematerial, particularly mineral matter, has been practiced for at least60 years in the United States utilizing the basic principle of applyingbody forces by the application of an electric field to a mixture ofcharged or polarized solid particles in such a manner that a selectivesorting of the particles will be effected dependent upon the degree ofand nature of the charge on the individual particles. Earlyelectrostatic separating devices generally comprised .machines forfeeding a mixture of mineral particles over a grounded conveying surfaceand applying an electrostatic field across the surface by means of ahigh tension electrode placed thereabove. Certain of the particles wouldbe attracted by the electrode and propelled over a divider bar while theremainder of the particles would be relatively unaffected by thepresence of an electrostatic field and would either remain on theconveying surface or be propelled under the divider bar. An early U.S.Pat.,No. 476,991, embodying this principle was granted to Mr. Thomas A.Edison in 1892.

The basic principles of Mr. Edisons electrostatic separator are usedeven today in the commercially adopted high tension machines andconductive induction machines wherein the particulate material ischarged either by the corona discharge of a high voltage electrode or byan electrical field placed adjacent to a grounded rotor carrying thefine dry particulate material. In the high tension devices, therelatively nonconductive particulate material tends to remain pinned tothe grounded rotor while the relatively conductive material isthrown-off due to centrifugal forces or due to an attracting electrodeplaced downstream from the corona discharge. In the conductive inductionmachines the relatively conductive material assumes the charge of therotor and is attracted in the electrostatic field while the relativelynon-conductive material remains on the rotor.

In addition to the machines hereinbefore mentioned wherein separation isperformed by lifting or propelling certain of the particles in a mixtureof particulate material a greater distance from a grounded surface thanthe rest of the particles, attempts have also been made to move materialover a horizontal or inclined surface and, by electrostatic means, causecertain of the material to move laterally of the remainder of thematerial against the gravitational or vibrational feed of the material.One of the earliest disclosures of such an apparatus is contained inU.S. Pat. No. 1,020,063 to Sutton et al. issued on Mar. 12, 1912. In oneembodiment of the electrostatic separator proposed by Sutton et al, analigned series of charged electrodes are provided extending parallel tothe direction of feed of particulate material on a conveying surface,and the electrodes are arranged to be moved in a direction laterally ofthe conveying surface so as to cause certain of the particles to alsohave a resultant direction of movement laterally of the direction offeed of the material. In another embodiment of the invention disclosedin the Sutton etal. patent, a plurality of spaced overhead electrodesare provided with a particular individual configuration wherein thegreatest electrical field density is concentrated at the ends of the.electrodes laterally of the main feed path of the material so that,again, certain of the particulate material is caused to move in adeflected path relative to the main flow of material.

In recent years renewed attempts have been made to electrostatically orelectrodynamically separate material which is moved along a generallyhorizontal conveying surface by causing a portion of the. material to bedeflected laterally of the main body of material. Use of, specialelectrode configurations, AC as well as DC electrical fields, andgrooves or deflecting barriers for aiding in the separation of materialhave all been utilized, sometimes in relatively complex ways, in orderto achieve a truly efficient and highly sensitive method of separatingdry particulate material. Examples of patents which disclose suchapparatus are U.S. Pat. Nos. 2,699,869 to Gear, No. 3,096,277 toMaestas, and No. 3,217,880 to Benton.

Another method of achieving separation of mineral material by theapplication of high voltage electrical fields is disclosed in U.S. Pat.No. 3,009,573 to Whipple. In the separator disclosed in this patentan'electrical field of sufficient intensity is provided so as to preventthe passage of certain particulate matter through the field-A series ofsuch electrical fields are presented to the particulate material whichis delivered vertically so as to normally gravitate through the fields.Certain of the material is levitated away from the fields and deflectedlaterally to a position where it can be separately collected.

SUMMARY OF THE INVENTION v The electric field separating apparatus ofthe present invention comprises i an improvement over those separatorsof the prior art wherein separating was attempted in a generallyhorizontal or inclined plane by deflecting certain particles laterallyof the direction of feed of the material. A passage for the flow ofparticulate material is provided and an electrical field is providedlaterally across this passage by providing electrodes spaced above andbelow and electrically insulated from the path of the material whichelectrodes are in alignment and extend laterally of the path of feed ofthe material. When the electrical field is applied by oppositelycharging the electrodes a certain portion of the mixed material willnormally be prevented from moving through the field and will be causedto move in a direction parallel to the field and laterally of thedirection of the remainder of the material which proceeds normallythrough the field in the feed direction. By adjusting the frequency andmagnitude of i the voltage applied to the electrodes, separations ofdifferent conductive or dielectric materials can be obtained. Ingeneral, relatively conductive materials are most easily separated fromrelatively non-conductive materials as in other electric fieldseparators; however,

by varying the frequency of the voltage, separations of relativelynoniconductive materials having similar dielectric constants may even beobtained,

In the preferred embodiment of the invention a series of parallel spacedelectrical fields are provided extending laterally of the-feed path ofthe material but at an angle thereto with at least a component thereofin the direction of propulsion of the material. These electrical fieldsare narrow so that a series of barriersare provided with a certainportion of the material being deflected along the upstream edge of eachof the electrical fields with the bulk of such portion of the materialbeing deflected by the first few electrical field barriers. An'ACvoltage is provided between the sets of electrodes spaced above andbelow the feed path, and it has been found that by varying the frequencyof this applied voltage, different materials can be caused to berepelled by the fields.

In contrast with the prior art structures, particle separation in thepresent invention is achieved positively at the upstream edges of theelectrical fields which are sharply defined-by an electrode gridstructure. With the separator of the present invention criticalseparations of mixed minerals such as chrysocolla and chalcopyrite canbe achieved which mineral mixture has not been effectively separatedhereinbefore; also, 'the separation of wolframite and monazite has beenachieved.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic sideelevation of the electric field separator of the present invention in anoperative assembly including the means for feeding the particulatematerial continuously to the separator.

FIG. 2 is an enlarged plan of the separator of FIG. 1 with a portion ofthe cover plate thereof being broken away for the purpose of clarity.

FIG. 3 is an enlarged side elevation of the apparatus of FIG. 2 withportions thereof being shown in section.

FIG. 4 is an exploded isometric illustration of the main structuralcomponents of the separator particularly illustrating the structure ofone of the electrode 7 grids.

FIG. 5 is an enlarged plan view of the conveyor plate and portions ofthe overlying plate and cover structure of the separator of FIG. 2particularly illustrating the location of the deflector members and thebarrier means with respect to the underlying electrode grid structure.

FIG. 6 is an enlarged isometric view of one of the deflector membersshown in FIG. 5.

FIG. 7 is an enlarged isometric view of a portion of the barrier meansand collecting means for the repelled material as shown in FIG. 5 withportions thereof being broken away for the purpose of clarity.

FIG. 8 is an enlarged, partially diagrammatic section taken along lines8-8 of FIG. 2 showing the dielectric plates between which theparticulate material is moved and particularly illustrating the natureof the electrical fields and their effect upon the material.

FIG. 9 is a schemmatic illustration of the electrical circuitry for theseparator of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The electric field separator 20of the present invention, as shown in an operative material processingassembly in FIG. I, is adapted to be utilized in conjunction with afeeding apparatus 22 continuously feeding fine particulate material M toone end of the separator. The particulate material is propelledlongitudinally along the separator in a horizontal direction and, if notrepelled by the electric fields, is discharged at the other end of theseparator upon a conveyor take-away belt 24. Particulate material whichis repelled by the electric fields will be deflected laterally by thefields and collected in troughs 26 arranged adjacent the side edges ofthe separator. A middling product containing a mixture of the twoseparations of the particulate material will be discharged at the end ofthe separator adjacent the discharge conveyor belt 24 but at the sidesthereof and into underlying collection containers 28. In accordance withstandard mineral separation procedures, the middling product collectedin the containers 28 will from time to time be transferred back to thefeeding apparatus for reprocessing through the separator.

The feeding apparatus 22 may comprise a conventional vibratory feeder orvibratory conveyor trough and hopper combination. The trough 30 isvibrated by a vibratory feeder structure 31 for vibratorily conveyingparticulate material which feeder structure includes a base 32 whichmounts the lower ends of a pair of flexible leaf springs 33 the otherends of which are connected to a mounting bracket 34 attached to thetrough. The mounting bracket is vibrated by an electromagnetic drivemotor 35. A hopper 37 containing a supply of the mixed particulatematerial M is attached at the reward end of the feeder trough, and, asthe trough is vibrated, the material will be incrementally moved out ofthe open bottomed hopper and along the face of the trough to bedeposited vertically upon the adjacent end of the separator 20. Thetrough is preferably provided with a heated liner (not shown) which willraise the temperature of the material to a sufficient degree before itis delivered to the separator so that the effects of ambient humidity orthe initial presence of latent moisture in the material will not beallowed to alter the electrical characteristics of the particles.

The separator, as shown in the exploded view of FIG. 4, basicallycomprises a mounting plate 38, a lower electrode plate 40 upon which thematerial is adapted to be conveyed, an upper electrode plate 42overlying the lower electrode plate with the undersurface thereof beingspaced from the top surface of the lower electrode plate, and a coverplate 44 which overlies the top electrode plate and which is securelyfastened to the side edges of the mounting plate by screws 45 (FIG. 3)to clamp the entire structure in place. The separator is driven by aconventional vibratory feeder 46, similar to the vibratory feeder orvibratory conveyor 31, including a base member 47 supporting the lowerends of a pair of leaf springs 48 the upper ends of which are rigidlyconnected to a mounting bracket 49 which is in turn, securely attachedtoa stiffner plate 52 that is connected to the underside of the mountingplate 38 (FIG. 3). The vibratory feeder is driven by an electromagneticdrive member 50 which is mounted upon the base member and includes anoscillating shaft connected to one end of themounting bracket 49.*Thevibratory I feeder is adapted to vibrate the entire separator struc'ture generally in an oscillatory path extending at an angle tothehorizontal so as to move the individual particles of the material ina progression of incremental steps from the feed end of the separator tothe discharge end thereof and down a sloping discharge plate 54 which isattached to the edge of the lower elecand bottom surfaces of the upperand lower electrode plates, respectively.

When the upper electrode plate is positioned over the lower electrodeplate it is usually preferable that the grids do'not directly overlieeach other in a vertical plane. Consequently, the grids on the topelectrode Electrode grid structures of matching configuration are Iprinted upon the top surface of the upper electrode plate 42 and uponthe bottom surface of the lower electrode plate 40 in order to provide aplurality of electrical fields which extend between the electrodeplatesand directly affect the movement of the materialupon the surface of thelower electrode plate. All of the structure of the electrostaticseparator which encomthe separator is the material polycarbonate whichhas no known fatigue life, excellent abrasion resistance, and a highdielectric strengthwhich allows for extremely high density electricfields that enable large (8-l0 mesh) particles to be separated.

I An important feature of the present'invention is the particulararrangement of the electrode grid structures which provide the.electrical fields. As best seen in FIGS. 4 and 5, the electrode gridstructures are of generally herringbone shape. The top electrode grid 60(FIG. 4) includes a central grid member 60a extending inthe direction'of vibratory movement of the material in the separator, and a pluralityof grid members 60b connected with the central grid on both sidesthereof and extendinglaterally at an angle thereto inclined in thedirection of movement of the material in the separator; The grid members60b are spaced a certain distance apart which, in the embodiment of theinven tion shown, is about one inch. At the outer ends of each of thegrid members 60b there is a break (FIG. 4), and sets of interconnectedgrid members 60c areprovided in alignment with the grid members 60b soas to form a pair of separate electrode structures at each side of theupper electrode plate. The bottom electrode grid structure 62 (show'nin' dashed linesin FIG. 5) is virtually identicalto the top electrodegrid structure 60 and includes a central grid member 62a and a pluralityof interconnected, laterally extending grid members 62b. Separateelectrode grid structures including grid members 620 are provided at thesides of the lower electrode plate. Preferably, the electrode gridstructures are comprised of a good conducting medium such as silverprint which can be sprayed or painted on the top plate are spacedslightly forward in the direction of movement of the material as isillustrated in FIG. 5 or,

particularly, in the enlarged sectional view of FIG. 8. Because of thisrelative positioning of the electrodes, the electrical fields, asindicated by the field line F in FIG. 8, will be inclined slightlyforward in the direction of movement of material between the electrodeplates (indicated by the arrow in FIG. 8) which inclination has beenfound to be more effective in achieving a separation of different.particulate matter. The upper electrode plate, in a manner to beexplained hereinafter, is adjustable upon the lower electrode plate sothat the relative positioning of the electrode grid structures in thehorizontal plane may be altered to virtually any desired condition.

, .Material which is fed to the lower electrode plate 40 l will bereceived between a back wall 70 and a front wall 72 in a pocket 74formed therebetween by short side wall sections 73.-The central loweredge of the front wall 72 is provided with a narrow opening 75 so thatmaterial within the pocket will be propelled forwardly upon the centralsurface of the lower electrode plate in a thin layer. As the materialmoves along the lower electrode plate a certain portion of the materialwhich is relatively-unaffected by the presence of the electricfieldswill rnove'forward generally in the direction of the arrows A(FIG. 5) while another portion of the material will be deflected andcaused to move in a direction parallel to the electrode grid members 62bas indicated by the arrows B (FIG. 5).

Positioned atopthelower electrode plate 40 arelocations overlying thespace between the grid members b and 60c are a plurality of deflectormembers 80, one of which is shown in detail in'FIG. 6. These deflectormembers bridge the gap between the electrode plates so that the'upperelectrode plate 42. rests upon the top surface 81 of the deflectormember. Each deflector member has a leg'82 extending generally in aninward'direction towards the grid member 62a so as toredirect certainparticles back toward the centerof the electrode plate which haveundesirably been moved laterally not directly as a result of theelectricfields but rather as a result of collision with other particles.Each deflector also includes a second leg 84 extending parallel to theunderlying electrode grid members 62b and 620 and bridging the gaptherebetween so that the particles which are repelled by the electricalfield generated between the grid members 60b and 62b will not bepermitted to move forwardly on the electrode plate 40 until they aresubjected to the influence of the electrical field generated between'thegrid members 60 c and 62c. In practice, it has been foundthatv materialwhich is'repelled by the'electrical fields'will be caused to move alonga line parallel to but somewhat rearwardly positioned from the gridmembers generally in the plane of the trailing face 86 of the deflectormember. This is illustrated somewhat diagrammatically in FIG. 8 whereinheavy concentrations of the material are caused to be formed along lineslocated rearwardly of the electrical fields which material willgradually move laterally in an inclined direction as a result of thecombination of the vibratory feeding forces and 'the repelling force ofthe fields.

Mounted upon the side portions of the top surface of the lower electrodeplate 40 area pair of barrier structures 90, shown in detail in FIG. '7,which provide a further separation of the particulate material. Aplurality of upright walls 92 extending between the electrode platesform physical extensions of the electrical fields. These walls areinterconnected by low barrier walls 94 over which the repelled materialmust pass in order to be received within pockets 96 formed at the sideedges of the lower electrode plate by the walls 94. Each of the pockets96 is provided with a vertical discharge passage 97 which extendsthrough the lower electrode plate and the underlying mounting plate 38(FIG. 3) so that material received within the pockets will be ultimatelydelivered to the underlying collecting troughs 26. Material which doesnot move over the low barrier walls 94 will eventually be propelled downthe face of the lower electrode plate and discharged into the collectingcontainers 28 for the middling product. In practicing the presentinvention, it has been found desirable to space the electrode plates bya distance of 3/ l 6ths of an inch and to make the barrier wall height.l/1'6th of an inch, or 55rd of the total height of the passage.

As shown in FIG. 4, the inclined discharge plate 54 at the downstreamend of the separator is provided with a pair of upright guide walls 101between which the non-repelled material is arranged to pass. To furtheraid in the separation of this material from the middling product, a pairof splitter bars 103 are pivotally mounted upon the downstream edge ofthe lower electrode plate 40. Each splitter bar is provided with a knifeedge 104 at its inwardly projecting end in order to make a cleandivision between'the particulate material traveling down the dischargeend of the lower electrode plate. The knife edge is positioned adjacentto the last of the deflector members at the end of the electrode gridstructure. By pivoting the splitter bars 103 inwardly, only the materialat the very center of the electrode plate will be separated from theremainder of the material, and the quality of this product may therebybe upgraded in certain instances, e.g., where the effect of theelectrical fields is only such as to cause a slight deflection ofcertain mixed-mineral matter.

The circuitry for applying an AC voltage of a variable frequency betweenthe upper and lower electrode grid structures 60 and 62 is showndiagrammatically in FIG. 9. A conventional audio oscillator 110 is usedto generate a variable frequency voltage including frequencies withinthe range of from about 30 cycles per second to about 400 cycles persecond. Transformer T1 connects the output of the audio oscillator to alinear amplifying circuit which separately amplifies each half cycle ofthe applied sine wave and transfers the amplified voltage to the primaryof a high voltage transformer T2, the secondary of which is directlyconnected to the electrode grid structures. Each half cycle of theapplied voltage is amplified by a cascaded threestage amplifier sectioncomprising transistors TR], TR2 and TR3 with the output of transistorTR3 being applied to the base of a power switching transistor TR4 whichis connected in series with the transformer primary. In order to aid inmaintaining the output linear with respect to the input from theoscillator 110, a pair of cascaded transistors TR5 and TR6 are connectedbetween ground and the emitter lead of each of the switching transistorsTR4. The amplifier sections are emitter biased by a positive voltage +Vthrough the biasing resistors R1 and R2 and are collector biased by anegative voltage -V, the negative voltage also serving to drive theswitching transistors TR4 through a grounded center tap on the highvoltage transformer primary. One end of the secondary of the highvoltage transformer includes leads L1, L2 and L3 which are connected tothe side electrode grid members 620 and to the center electrode gridmember 620, respectively, on the lower electrode plate as shown in FIG.5. The other end of the secondary is connected to the center electrodegrid member 60a on the upper electrode plate through a lead L4 extendingwithin an upright insulator post 112 (FIG. 3). A variable voltage tap isprovided in the secondary of the high voltage transformer T2 andincludes a pair of leads L5 and L6 which are connected to the sideelectrode grid members 60c through upright insulator posts 1 14.

It will be noted that the voltage between the overlying side electrodegrid members 600 and 620 will be less than that between the overlyingcenter electrode grid members 60b and 62b. Thus, the tendency toseparate two dissimilar materials will be decreased, i.e., made moresensitive, in the side sections of the separator, and a furtherseparation of material can be made in the side sections in order toobtain a high quality concentrate in the collecting troughs 26.

' In assembling the separator of the present invention, the upperelectrode plate 42 is placed upon the top of the deflector members andthe barrier structures on the lower electrode plate 40. The cover 44 isthen placed over the top electrode plate with elongated slots 116 at oneend thereof being received over the upright insulator posts 114 and 112connecting the electrode structures to the transformer. The cover isthen securely fastened to the mounting plate 38 by the screw 45 whichserves to clamp the electrode plates tightly together. Since the slots116 are elongated it will be apparent that the upper electrode plate canbe slid horizontally with respect to the lower electrode plate beforethe plates are clamped together. In practicing the present invention, itis normally desirable to have the electrode grid structure on the upperelectrode plate spaced slightly forward of the electrode grid structureon the lower electrode plate as pointed out previously. The angle ofinclination thus imparted to the electrical fields can be varied toachieve different separationeffects, and, in some instances, the topelectrode grid structure may directly overlie the lower elec trode gridstructure in order to obtain the maximum effectiveness in separatingcertain minerals.

While the theory of operation of the electric field separator of thepresent invention is not entirely understood, it is believed that thedegree of separation of any mixture of different particles primarilydepends upon the dielectric properties of the material, the particlesize and density, the net charge upon the surface of the particle, theintensity and frequency of the varying electrical fields applied to thematerial, the feed rate, and the importance of deflections due to randomcollision with other particles.

in general, the larger the particle size the less the degree ofactivity, i.e., the less the particle will be effected by the electricalfields. This factor can be directly compensated for by increasing theelectrode voltage; therefore, for larger article sizes the voltagebetween the electrodes must be increased accordingly. Obviously thisfactor imposes a upper limit upon the particle size for any givenseparator structure since the electrode voltage can only be increased toa certain maximum amount before dielectric breakdown occurs. Whenparticles in a mixture are of different size groups, it is the normalpractice to first separate the particles into different size groups andto process each group separately since the operating voltage during anyone run of the separator will only effectively separate particles of agiven size range.

The density factor appears to effect the distance that the particles arerepelled from the electrical field. Hence, as the density of theparticles varies it may be necessary to vary the horizontal position ofthe upper electrode plate with respect to the lower electrode plate inthe manner hereinbefore explained to thereby vary the inclination of theelectrical field in the path of movement so that the main body ofrepelled particles will be able to be moved laterally between thedeflector members 80 without being physically impeded.

The feed rate of the material across the separator is important from twostandpoints, the volume of material moved per unit area and the speed atwhich the particles are propelled. The volume of the feed is importantin that it theoretically should provide for all of the particles to beseparated by the time they reach the discharge end of the separator. Ifthe volume feed rate is too high, a particle which would normally bedeflected by the electrical fields may be physically forced through thefields due either to random collision with the other particles or to themasking effects encountered when there is a high concentration ofparticles in the area of the fields. The feed rate also has a directbearing on the frequency of variation of the electrical fields, aspreviously pointed out. Considered in another manner, it can be seenthat a high feed rate means a higher propelling force upon the particleswhich will tend to offset the deflecting forces due to the presence ofthe electrical fields.

Particles to be separated can generally be grouped into twoclassesactive and non-reactive. Active particles can be defined as theparticles which will be deflected by the electrical fields, andnon-reactive particles can be defined as those particles will movethrough the fields. Whether or not particles which can be classified asactive or non-reactive will depend upon the frequency and intensity ofthe applied AC voltage as well as the particular mixture of particlesbeing separated. As the frequency is varied certain particles which wereformerly non-reactive become active and vice versa.

It has been found that a mixture of chalcopyrite and chrysocollaparticles at a particle size of between 65 and 100 mesh can be separatedwith the separator of the present invention by applying a frequency ofalternation of the electrical fields of about 162 cycles per second.Examples of non-reactive particles are those with usually perfect ornear perfect crystalline structures, such as galena, quartz crystals,and some varieties of garnet and tourmaline. Active and non-reactiveparticles in fields generated by AC voltages of five to ten thousandvolts with a frequency of 60 cycles per second can roughly be classifiedinto two groups as follows:

Active Particles Non-active Particles 1. All micaceous minerals l.Calcite 2. Feldspar 2. Scheelite 3. Chalcopyrite 3. Apatite 4. Pyrite 4.Fluorite 5. Cobaltite 5. Azurite 6. Magnetite 6. Malachite 7. Hematite7. Barite 8. Rutile 8. Limonite 9. Gold (native) 9. Zircon 10. Silver(native) l0. Garnet l 1. Gold (tellurides) l l. Sulphur 12. Silver(sulphides) l2. Monazite 13. Diamond 13. Cerussite 14. wolframite l4.Tourmaline 15. Cinnabar l5. Psilomelane 16.Thorite 16. Beryl 17.Molybdenite 17. Cassiterite l8. Chrysocolla l8. Sphalerite (all butMarmatite) 19. Copper (native) 20. Sphalerite (black).

21. lllmenite It must be remembered that these groups are forillustrative purposes only, and some particles in the active group maybecome non-reactive, or vice versa, depending upon the particle size andthe particular mixture of particles being separated.

As an example of a separation that can be accomplished with theseparator 20 of the present invention, a mixture of 20.7 percentwolframite and 79.3 percent monazite was processed at an effective feedrate of approximately 30 pounds per hour between electrode plates of 10X 16 inches or roughly one square foot in area. A voltage ofapproximately 6,000 volts was utilized between the center electrode gridstructures and approximately 4,500 volts between each of the sideelectrode grid structures at a frequency of 60 cycles per second. Afterone pass through the separator 97.6 percent of the wolframite wasdeflected by the electrical fields and collected at thesides of theseparator while another 1.4 percent was found in the middling productcollected at the discharge ends of the separator. It can therefore besaid that a total of 99 percent of the wolframite was recovered sincethe middling product would normally have been recycled through theseparator. The concentrate separated at the sides of the separator wasfound to be 99 percent wolframite with the original wolframitepercentage being only 20.7 per cent of the total material processed.

Feed rates of up to 100 pounds per hour can be achieved in a separatoras disclosed with electrode plates of approximately l0 inches by 16inches..- Furthermore, while the separator of the present invention isdisclosed as including only a single separating zone, it will berecognized that various separating zones may be provided one on top ofthe other with the entire structure being driven by a single vibratoryfeeding device. That is to say, the upper electrode plate 42 may beplaced below a second lower electrode plate 40 which, in turn, may haveanother upper electrode plate 42 placed thereon, and so forth, until aconsiderable number of material feeding channels are thus provided invertically spaced positions. in this way, the feed capacity of theseparatormay be multiplied while the power requirements and spacerequirements are increased only slightly.

Multiple separations of the material can be achieved by isolatingindividual or, preferably, sets of electrode grid members 60b and 62bspaced along the length of the separator. AC voltages of differentfrequencies are then applied to each set of grid members. Since it hasbeen found that particles may be rendered active or non-reactive inaccordance with the frequency of the applied electrical field, it can beseen that different particles might be repelled at differentlongitudinal positions in the separator and that a progressiveseparation of several mineral materials is possible with but a singlepass through the separator. It is also possible to increase the numberof separations in a single pass by utilizing gravitational effects inaddition to the electric field effects as by tilting the electrodeplates about their longitudinal axis and/or using a riffled conveyingsurface for example.

It can be seen that the electric field separator of the presentinvention comprises a highly efficient apparatus which has been found tobe very effective for separating mineral material from mesh to 500 meshwhich would other wise be difficult if not impossible to separate byconventional electrostatic or other electric field separatingprocedures. For example, separations of cassiterite from pyrite,monazite from euxenite, sheelite from wolframite, cerussite from pyrite,and garnet from magnetite and illmenite are possible. The presentinvention can therefore be used to clean and separate various fractionsderived from conventional gravity, flotation and electric mineralseparating processes. Furthermore, a great degree of flexibility ispresent in the device of the present invention since the magnitude andfrequency of the applied voltage may be varied to vary the effects ofthe electrostatic fields upon the different mineral particles. Inaddition, the location of the fields relative -to the path of movementof the particles is easily changed by shifting the relative horizontalpositions of the electrode plates.

Although thebest mode contemplated for carrying out the presentinvention has been herein shown and described, it will be apparent thatmodification and variation may be made without departing from what isregarded to be the subject matter of the invention.

What is claimed is:

l. A method of separating fine particulate material of which a portionof said material is attracted to an electrical field to a differentdegree than the remainder of said material, said method comprising thesteps of providing an electrical field which extends in a generallyvertical plane by applying an electric potential between a pair ofspaced electrodes, moving said material along a path at least acomponent of which lies in the horizontal direction and into and out ofsaid electrical field, said electrodes being spaced above and below saidpath and electrically insulated from said path and which electrodesextend transversely to said path, and moving that material which doesnot pass through said electrical field along said electrical field in adirection generally parallel to the electrodes, whereby a portion ofsaid material is caused to move laterally of said path in a directionparallel to said electrical field without moving across said field whilethe remainder of said material will be moved across said electricalfield generally in the direction of said path.

2. A method of separating fine particulate material according to claim 1wherein a plurality of parallel electrical fields are provided with saidfields extending laterally of said path and being spaced in thedirection of said path.

3. A method of separating fine particulate material according to claim 1including the step of providing a second electrical field aligned end toend with the first electrical field in the direction of movement of saidportion of said material, said second electrical field being of a lowerintensity than the first electrical field.

4. A method of separating fine particulate material according to claim 3including the step of redirecting material toward the center of saidpath in the area between said first and second electrical fields.

5. A method of separating fine particulate material according to claim 2including the step of further separating said remainder of said materialat the downstream end of said path by separating the material at a sideof the path from the material in the central portion of the path.

6. A separator for separating fine particulate material comprising amaterial conveyor having a conveying surface at least a component ofwhich lies in a horizontal plane, means for conveying said material in afirst direction along said conveying surface, a pair of electrodesextending in generally parallel planes and in a second directiontransversely to said first direction, said electrodes being spaced aboveand below said conveying surface and being electrically insulated fromsaid conveying surface, and means for applying a voltage between saidelectrodes to provide an electrical field across said conveying surfacewith the conveying surface extending both upstream and downstream ofsaid electrical field whereby a portion of said material will bedeflected by said electrical field and caused to move in said seconddirection while the remainder of said material moves through saidelectrical field in said first direction.

7. A separator for separating fine particulate material as set forth inclaim 6 wherein said second direction has a component thereof parallelto said first direction.

8. A separator for separating fine particulate material as set forth inclaim 7 including a pair of sets of electrodes spaced above and belowsaid conveying surface, each of said sets of electrodes comprising aplurality of parallel electrodes extending in said second direction andbeing spaced apart in said first direction.

9. A separator for separating fine particulate material as set forth inclaim 8 wherein said conveying surface lies in a horizontal plane andwherein said means for conveying said material comprises a vibratoryconveymg means.

10. A separator for separating fine particulate material as set forth inclaim 8 wherein said voltage is conveyor comprises a first flat plate ofdielectric material having one of said sets of electrodes provided onthe undersurface thereof and said conveying surface being provided onthe flat top surface thereof and a second flat plate of dielectricmaterial spaced above said first plate and having the other of said setsof electrodes provided on the top surface thereof.

13. A separator for separating. fine particulate material as set forthin claim 12 including a second pair of sets of electrodes with one setof said second pair being provided on the undersurface of said firstflat plate adjacent to and in alignment with the electrodes of the firstset but spaced therefrom in said second direction and with the other setof said second pair being provided on the top surface of said secondflat plate adjacent to and in alignment with the electrodes of the firstset but spaced therefrom in said second direction, and means forproviding a voltage between the electrodes of said second pair of setsof electrodes which voltage is lower than that between said first pairof sets of electrodes.

14. A separator for separating fine particulate material as set forth inclaim 13 including a plurality of deflector means provided between saidflat plates in positions overlying the space between said first andsecond sets of electrodes for deflecting a certain portion of saidmaterial generally in said first direction and away from said seconddirection.

15. A separator for separating fine particulate material as set forth inclaim 13 including a plurality of barrier means provided upon the topsurface of said first flat plate in positions overlying the outermostends of said second set of electrodes, and means positioned outwardly ofsaid barriers for collecting that portion of said material which iscaused to move over said barrier means.

16. A separator for separating fine particulate material comprising aconveying member having a conveying surface with at least a componentthereof extending in a horizontal direction, means for moving saidmaterial in incremental movements in a longitudinal direction along saidconveying surface, a first electrode grid structure positioned belowsaid conveying surface, said grid structure being electrically insulated from said conveying surface and including a V- shaped grid memberwith the legs extending from the apex of the Vin said longitudinaldirection of movement of said material and being equiangularly locatedwith respect to said longitudinal direction, a second electrode gridstructure similar to said first electrode grid structure, said secondelectrode grid structure being positioned above said conveying surfacein alignment with said first electrode grid structure, and means forapplying a voltage between said electrode grid structures to create anelectrical field with the conveying surface extending both upstream anddownstream of said electrical field whereby a portion of said materialwill be deflected and caused to move laterally of said longitudinaldirection and along the upstream edges of said electrical field.

17. A separator for separating fine particulate material as set forth inclaim 16 wherein each of said electrode grid structures comprises aplurality of parallel V-shaped grid members spaced in said longitudinaldirection and with the legs thereof extending from the apices of the Vsin said longitudinal direction of movement of said material, the apicesof said grid members being interconnected by a further grid memberextending in said longitudinal direction with the legs of said V- shapedgrid members being equiangularly positioned with respect to said furthergrid member.

18. A separator for separating fine particulate material as set forth inclaim 16 wherein said voltage is an AC voltage with a variable frequencyrange of from about to about 400 cycles per second.

19. A separator for separating fine particulate material as set forth inclaim 17 including adjustment means for moving the electrode gridstructures relative to one another in said longitudinal direction so asto alter the inclination of the electrical fields provided thereby withrespect to a horizontal plane.

20. A separator for separating fine particulate material as set forth inclaim 18 wherein said conveying member comprises a flat plate ofdielectric material having said first electrode grid structure providedupon the undersurface thereof, and a second flat plate of dielectricmaterial overlying said conveying member in parallel relationshipthereto, said second flat plate having said second electrode gridstructure provided upon the upper surface thereof.

21. A separator for separating fine particulate material as set forth inclaim 17 including third electrode grid structures provided adjacent tothe lateral edges of said first electrode grid structure, said thirdelectrode grid structures including a plurality of interconnected gridmembers aligned end to end with but spaced from the grid members of saidfirst electrode grid structure, fourth electrode grid structures similarto said third electrode grid structures provided adjacent to but spacedfrom the lateral edges of said second electrode grid structure and inalignment with said third electrode grid structures, and means forproviding a voltage between said third and fourth electrode gridstructures independent of the voltage between said first and secondelectrode grid structures.

22. A separator for separating fine particulate material as set forth inclaim 21 including a plurality of deflector means provided upon saidconveying member in positions generally overlying the space between thegrid members ofsaid first and said third electrode grid structures fordeflecting a certain portion of said material generally toward aposition between said further grid members of said first and said secondelectrode grid structures.

23. A separator for separating fine particulate material as set forth inclaim 21 including a plurality of barrier means provided upon saidconveying member in positions overlying the outer ends of the gridmembers of said third electrode grid structures, and means positionedoutwardly of said barrier means for collecting that portion of saidmaterial which is caused to move over said barrier means.

24. A separator for separating fine particulate material as set forth inclaim 20 including a pair of splitter bars pivotally mounted upon thedownstream edge of said conveying member adjacent the side edges thereoffor separating the material conveyed along said conveying member into afirst portion moving along the central section of the conveying memberand a second portion moving along the sides of the conveying member.

25. A separator for the separation of fine particulate materialcomprising a pair of flat plates of dielectric material arranged inparallel spaced relationship in a generally horizontal plane, means forconveying said material between said plates in incremental movementsalong the upper surface of the lowermost plate in a longitudinaldirection, a first electrode provided above the uppermost plate, asecond electrode provided below the lowermost plate, said electrodesbeing mounted in parallel relationship to each other and extendingtransversely of said plates but inclined in said longitudinal direction,said conveying means being effective to move said material up to andover said second electrode, and means for providing a voltage betweensaid electrodes for providing an electrical field across said plates ofan intensity whereby a portion of said material is prevented frompassing through said electrical field in said longitudinal direction andis caused to move laterally along the upstream edge of said electricalfield.

26. A separator for the separation of fine particulate material as setforth in claim 25 including a plurality of said first and said secondelectrodes with said first electrodes being arranged in parallelrelationship in a generally horizontal plane and with said secondelectrodes being similarly arranged in parallel relationship in agenerally horizontal plane in alignment with the said first electrodeswhereby a plurality of spaced parallel electrical fields are providedbetween said plates extending transversely thereof.

27. A separator for the separation of fine particulate material as setforth in claim 25 wherein said voltage is an AC voltage of apredetermined frequency.

28. A separator for the separation of fine particualte material as setforth in claim 27 including means for varying the frequency of said ACvoltage in the range of from about 30 cycles per second to about 400cycles per second.

2353 UNITED STATES PATENT oEElcE CERTIFICATE OF CORRECTION Patent No. 72312 Dated M h 13, 1973 I fl PAUL R. SHOOK. EARL G. SWEENEY, RALPH E.DUNCAN It is certified that error appears in the above-identified patentand that said Letters Patent are hereby corrected as shown below:

r Column 4, line 29, after feeder insert or vibratory "T conveyor,-.Column 4, line 60, after feeder insert --or vibratory conveyor' Column4, line 61, delete "or vibratory conveyor". Column 9, line 5, "articleshould .be -particle-.

Column 9, line 50, delete the comma after "fields". Column 9, line 51,after particles insert which-. Column 9, line 52 after particles delete"which". Column 11, line 37, "electrostatic" should be ---'eleCtric--.Column 16, line 28, "particualte should be -.part:icul ate--. I 1 ISigned and sealed this 3rd day of. December 1974.

(SEAL) Attest:

:McCOY M. GIBSON JR. C. MARSHALL DANN Attesting Officer Commissioner ofPatents

1. A method of separating fine particulate material of which a portionof said material is attracted to an electrical field to a differentdegree than the remainder of said material, said method comprising thesteps of providing an electrical field which extends in a generallyvertical plane by applying an electric potential between a pair ofspaced electrodes, moving said material along a path at least acomponent of which lies in the horizontal direction and into and out ofsaid electrical field, said electrodes being spaced above and below saidpath and electrically insulated from said path and which electrodesextend transversely to said path, and moving that material which doesnot pass through said electrical field along said electrical field in adirection generally parallel to the electrodes, whereby a portion ofsaid material is caused to move laterally of said path in a directionparallel to said electrical field without moving across said field whilethe remainder of said material will be moved across said electricalfield generally in the direction of said path.
 1. A method of separatingfine particulate material of which a portion of said material isattracted to an electrical field to a different degree than theremainder of said material, said method comprising the steps ofproviding an electrical field which extends in a generally verticalplane by applying an electric potential between a pair of spacedelectrodes, moving said material along a path at least a component ofwhich lies in the horizontal direction and into and out of saidelectrical field, said electrodes being spaced above and below said pathand electrically insulated from said path and which electrodes extendtransversely to said path, and moving that material which does not passthrough said electrical field along said electrical field in a directiongenerally parallel to the electrodes, whereby a portion of said materialis caused to move laterally of said path in a direction parallel to saidelectrical field without moving across said field while the remainder ofsaid material will be moved across said electrical field generally inthe direction of said path.
 2. A method of separating fine particulatematerial according to claim 1 wherein a plurality of parallel electricalfields are provided with said fields extending laterally of said pathand being spaced in the direction of said path.
 3. A method ofsepaRating fine particulate material according to claim 1 including thestep of providing a second electrical field aligned end to end with thefirst electrical field in the direction of movement of said portion ofsaid material, said second electrical field being of a lower intensitythan the first electrical field.
 4. A method of separating fineparticulate material according to claim 3 including the step ofredirecting material toward the center of said path in the area betweensaid first and second electrical fields.
 5. A method of separating fineparticulate material according to claim 2 including the step of furtherseparating said remainder of said material at the downstream end of saidpath by separating the material at a side of the path from the materialin the central portion of the path.
 6. A separator for separating fineparticulate material comprising a material conveyor having a conveyingsurface at least a component of which lies in a horizontal plane, meansfor conveying said material in a first direction along said conveyingsurface, a pair of electrodes extending in generally parallel planes andin a second direction transversely to said first direction, saidelectrodes being spaced above and below said conveying surface and beingelectrically insulated from said conveying surface, and means forapplying a voltage between said electrodes to provide an electricalfield across said conveying surface with the conveying surface extendingboth upstream and downstream of said electrical field whereby a portionof said material will be deflected by said electrical field and causedto move in said second direction while the remainder of said materialmoves through said electrical field in said first direction.
 7. Aseparator for separating fine particulate material as set forth in claim6 wherein said second direction has a component thereof parallel to saidfirst direction.
 8. A separator for separating fine particulate materialas set forth in claim 7 including a pair of sets of electrodes spacedabove and below said conveying surface, each of said sets of electrodescomprising a plurality of parallel electrodes extending in said seconddirection and being spaced apart in said first direction.
 9. A separatorfor separating fine particulate material as set forth in claim 8 whereinsaid conveying surface lies in a horizontal plane and wherein said meansfor conveying said material comprises a vibratory conveying means.
 10. Aseparator for separating fine particulate material as set forth in claim8 wherein said voltage is an AC voltage of a predetermined frequency.11. A separator for separating fine particulate material as set forth inclaim 10 including means for varying the frequency of said AC voltage.12. A separator for separating fine particulate material as set forth inclaim 8 wherein said material conveyor comprises a first flat plate ofdielectric material having one of said sets of electrodes provided onthe undersurface thereof and said conveying surface being provided onthe flat top surface thereof and a second flat plate of dielectricmaterial spaced above said first plate and having the other of said setsof electrodes provided on the top surface thereof.
 13. A separator forseparating fine particulate material as set forth in claim 12 includinga second pair of sets of electrodes with one set of said second pairbeing provided on the undersurface of said first flat plate adjacent toand in alignment with the electrodes of the first set but spacedtherefrom in said second direction and with the other set of said secondpair being provided on the top surface of said second flat plateadjacent to and in alignment with the electrodes of the first set butspaced therefrom in said second direction, and means for providing avoltage between the electrodes of said second pair of sets of electrodeswhich voltage is lower than that between said first pair of sets ofelectrodes.
 14. A separator for separating fine particulatE material asset forth in claim 13 including a plurality of deflector means providedbetween said flat plates in positions overlying the space between saidfirst and second sets of electrodes for deflecting a certain portion ofsaid material generally in said first direction and away from saidsecond direction.
 15. A separator for separating fine particulatematerial as set forth in claim 13 including a plurality of barrier meansprovided upon the top surface of said first flat plate in positionsoverlying the outermost ends of said second set of electrodes, and meanspositioned outwardly of said barriers for collecting that portion ofsaid material which is caused to move over said barrier means.
 16. Aseparator for separating fine particulate material comprising aconveying member having a conveying surface with at least a componentthereof extending in a horizontal direction, means for moving saidmaterial in incremental movements in a longitudinal direction along saidconveying surface, a first electrode grid structure positioned belowsaid conveying surface, said grid structure being electrically insulatedfrom said conveying surface and including a V-shaped grid member withthe legs extending from the apex of the V in said longitudinal directionof movement of said material and being equiangularly located withrespect to said longitudinal direction, a second electrode gridstructure similar to said first electrode grid structure, said secondelectrode grid structure being positioned above said conveying surfacein alignment with said first electrode grid structure, and means forapplying a voltage between said electrode grid structures to create anelectrical field with the conveying surface extending both upstream anddownstream of said electrical field whereby a portion of said materialwill be deflected and caused to move laterally of said longitudinaldirection and along the upstream edges of said electrical field.
 17. Aseparator for separating fine particulate material as set forth in claim16 wherein each of said electrode grid structures comprises a pluralityof parallel V-shaped grid members spaced in said longitudinal directionand with the legs thereof extending from the apices of the Vs in saidlongitudinal direction of movement of said material, the apices of saidgrid members being interconnected by a further grid member extending insaid longitudinal direction with the legs of said V-shaped grid membersbeing equiangularly positioned with respect to said further grid member.18. A separator for separating fine particulate material as set forth inclaim 16 wherein said voltage is an AC voltage with a variable frequencyrange of from about 30 to about 400 cycles per second.
 19. A separatorfor separating fine particulate material as set forth in claim 17including adjustment means for moving the electrode grid structuresrelative to one another in said longitudinal direction so as to alterthe inclination of the electrical fields provided thereby with respectto a horizontal plane.
 20. A separator for separating fine particulatematerial as set forth in claim 18 wherein said conveying membercomprises a flat plate of dielectric material having said firstelectrode grid structure provided upon the undersurface thereof, and asecond flat plate of dielectric material overlying said conveying memberin parallel relationship thereto, said second flat plate having saidsecond electrode grid structure provided upon the upper surface thereof.21. A separator for separating fine particulate material as set forth inclaim 17 including third electrode grid structures provided adjacent tothe lateral edges of said first electrode grid structure, said thirdelectrode grid structures including a plurality of interconnected gridmembers aligned end to end with but spaced from the grid members of saidfirst electrode grid structure, fourth electrode grid structures similarto said third electrode grid structures provided adjacEnt to but spacedfrom the lateral edges of said second electrode grid structure and inalignment with said third electrode grid structures, and means forproviding a voltage between said third and fourth electrode gridstructures independent of the voltage between said first and secondelectrode grid structures.
 22. A separator for separating fineparticulate material as set forth in claim 21 including a plurality ofdeflector means provided upon said conveying member in positionsgenerally overlying the space between the grid members of said first andsaid third electrode grid structures for deflecting a certain portion ofsaid material generally toward a position between said further gridmembers of said first and said second electrode grid structures.
 23. Aseparator for separating fine particulate material as set forth in claim21 including a plurality of barrier means provided upon said conveyingmember in positions overlying the outer ends of the grid members of saidthird electrode grid structures, and means positioned outwardly of saidbarrier means for collecting that portion of said material which iscaused to move over said barrier means.
 24. A separator for separatingfine particulate material as set forth in claim 20 including a pair ofsplitter bars pivotally mounted upon the downstream edge of saidconveying member adjacent the side edges thereof for separating thematerial conveyed along said conveying member into a first portionmoving along the central section of the conveying member and a secondportion moving along the sides of the conveying member.
 25. A separatorfor the separation of fine particulate material comprising a pair offlat plates of dielectric material arranged in parallel spacedrelationship in a generally horizontal plane, means for conveying saidmaterial between said plates in incremental movements along the uppersurface of the lowermost plate in a longitudinal direction, a firstelectrode provided above the uppermost plate, a second electrodeprovided below the lowermost plate, said electrodes being mounted inparallel relationship to each other and extending transversely of saidplates but inclined in said longitudinal direction, said conveying meansbeing effective to move said material up to and over said secondelectrode, and means for providing a voltage between said electrodes forproviding an electrical field across said plates of an intensity wherebya portion of said material is prevented from passing through saidelectrical field in said longitudinal direction and is caused to movelaterally along the upstream edge of said electrical field.
 26. Aseparator for the separation of fine particulate material as set forthin claim 25 including a plurality of said first and said secondelectrodes with said first electrodes being arranged in parallelrelationship in a generally horizontal plane and with said secondelectrodes being similarly arranged in parallel relationship in agenerally horizontal plane in alignment with the said first electrodeswhereby a plurality of spaced parallel electrical fields are providedbetween said plates extending transversely thereof.
 27. A separator forthe separation of fine particulate material as set forth in claim 25wherein said voltage is an AC voltage of a predetermined frequency.